1. Field of the Invention
This invention relates to single facers for manufacturing single-faced corrugated paper and more particularly to improved construction of the rolls employed in single facers.
2. Description of the Prior Art
Much progress has been made in recent years in techniques for speeding up the manufacturing operation for making corrugated paper. A single facer (a machine for manufacturing single-faced corrugated paper) is one of the important machines employed in a corrugated paper manufacturing operation and one in which a higher speed of operation is desirable. In achieving such higher speed operation it is necessary to overcome a number of technical problems. One such problem is that to achieve a higher speed operation wherein the paper moves more rapidly over the surface of the corrugating rolls and the pressure roll of a single facer, it is necessary that heat be transferred more rapidly from the interior of the roll, to which heated vapor is normally supplied, to the exterior surface of the roll so as to adequately heat the paper. In the usual single facer, high pressure heated vapor is introduced into the interior of the hollow upper and lower corrugating rolls and the hollow pressure roll to heat a core sheet or a liner moving in contact with the outer circumferential surfaces of these rolls, to thereby improve the corrugating capability and the pasting and bonding capability of the rolls. As the speed of the single facer is increased and, hence, the speed with which the core sheet and liner pass over the surface of the rolls is increased, the amount of heat that can be transferred to a given section of paper moving over the roll through a wall of given thickness is reduced. The rate of heat transfer can be increased to correspond to the increased speed of the paper over the exterior surface of the rolls by reducing the thickness of the wall of each of the rolls. Unfortunately, it has been found that reducing the thickness of the rolls to accomplish greater heat transfer gives rise to another problem, namely, that the thinner-walled rolls tend to have an increased amount of vibration and an increase in noise transmitted thereby. Thus, this decrease in thickness of the wall of the hollow rolls resulted in the occurrence of a metallic noise of increased loudness. It has heretofore been considered that the noise generated by a single facer results mainly from the vibration of a rigid body, that is, vibration occurring when a bending vibration load on a roll generated as a result of a driving load between rolls works as a returning force generated by a roll-supporting bearing which functions as a spring. The occurrence of metallic noise of increased loudness referred to above was analyzed to discover that the noise level increases were due to elastic vibration rather than vibration of a rigid body as discussed above. This elastic vibration means vibration occurring on the basis of the deformation of the roll itself. The higher noise level, therefore, can be ascribed to an increase in the level of elastic vibration of the thinner roll shells and an increase in the level of high frequency vibration caused thereby. The increase in the loudness of noise occurring during a corrugating operation using rolls having walls of reduced thickness is particularly noticeable in the region of a high-speed operation.
The nature of the generated noise may be better appreciated by reference to FIG. 1 which illustrates a so-called Campbell's diagram showing the results of analysis of actual measurements of noise generated by a single facer having a pressure roll with a diameter of 500 mm, a length of 2,200 mm, and a roll wall thickness of 25 mm. This diagram shows the amplitude of the noise by the diameter of the circle for each noise frequency and for overall range of meshing frequency, which is generated when the tips of the corrugations on the lower corrugating roll are beaten by the pressure roll.
As may be noted from this diagram, the noise level becomes high at specific noise frequencies, for example, around 300-500 Hz, 800 Hz, 1500 Hz. These frequencies agree with the natural frequencies (in a mode of K=1, N=2-4) of a hollow cylindrical shell, as shown in FIG. 2. This means that metallic and discordant noise occurs when natural vibration in the above-mentioned mode takes place due to the meshing frequency NZ and its higher harmonics. The natural frequency of a hollow cylindrical shell is generally decreased as the thickness of the shell is decreased, but, at the higher or second resonance speed, that is at the second harmonic, a larger amplitude occurs depending on the changing damping factor, and this yields undesirable metallic noise. Therefore, attempts to improve the heat transfer rate of a roll by conventional technique to increase the speed of operation, that is, by reducing the thickness of the wall of the roll, gives rise, for the above-discussed reasons, to a problem of an increase in the loudness of the noise.
It is an object of the present invention to provide a single facer having an upper corrugating roll, a lower corrugating roll and a pressure roll so constructed that rolls having thinner walls for greater heat transfer may be employed, but further constructed so that the noise transmitted during the operation of the single facer is minimized.
It is a further object of the present invention to minimize the elastic vibration of the rolls and thereby minimize the noise resulting from such elastic vibration.